Purification of gases in air may be useful or vital in enclosed environments such as spacecraft, space habitats, submarines, underground mines, and terrestrial and non-terrestrial vehicles (e.g., aircraft, armored vehicles, and pressurized rovers). In particular, people produce carbon dioxide (CO2) as a metabolic byproduct that can become noxious if accumulated within an enclosure. The average person exhales almost a kilogram (kg) of carbon dioxide per day. In environments containing higher levels of carbon dioxide, people may experience symptoms such as nausea, dizziness, and headaches. Hence, manned, enclosed environments need a mechanism to remove carbon dioxide produced within the environment. Additionally, animals and plants are sensitive to the level of carbon dioxide and would benefit from carbon dioxide control.
Two methods are commonly used to control the carbon dioxide level in manned spacecraft—adsorption and chemical reaction. Chemical reaction systems use chemicals (such as lithium hydroxide or potassium superoxide) that react with carbon dioxide in the air to form benign products. These systems are one-time use. Once all of the chemical is reacted, no more carbon dioxide may be removed from the air. Chemical reaction systems may use relatively little electrical power (e.g., to operate a blower to direct air through a bed of the carbon dioxide-reactive chemical) but do require a significant supply of replacement chemical. For example, a lithium hydroxide system requires about 1.5 kg of lithium hydroxide to remove the carbon dioxide produced by one person per day.
Adsorption systems are regenerative and thus do not have significant supply requirements. Adsorption systems selectively adsorb carbon dioxide under certain conditions and release carbon dioxide under other conditions. For example, the International Space Station uses beds of zeolite pellets that adsorb carbon dioxide from the air of the cabin and that release the carbon dioxide when heated under vacuum (e.g., exposed to space vacuum). Because zeolites commonly preferentially adsorb water vapor over carbon dioxide, zeolite-based systems typically include a regenerative desiccant (e.g., a bed of silica gel) to remove water from the air to be exposed to the zeolites. Adsorption systems may use significant electrical power to regenerate the adsorbent bed and/or the desiccant bed. Additionally, adsorption systems may be quite bulky and heavy. For example, the carbon dioxide removal systems on the International Space Station require about 1 kW (kilowatt) of power and the adsorbent beds weigh about 48 kg each.